Catalytic converters comprising a three-way catalyst, etc. are disposed in the exhaust passages of internal combustion engines for purifying unburned gases such as hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), etc. contained in exhaust gases emitted from the engines based on catalytic actions (oxidizing and reducing actions) of the catalytic converters. The catalytic converter of the type described is, however, not sufficiently activated when its temperature is low, such as when the internal combustion engine starts to operate at a low temperature. When the catalytic converter is not sufficiently activated, a large amount of hydrocarbons in particular is emitted from the exhaust emission system.
It is known in the art to place an exhaust gas purifier disposed in the exhaust passage, which may comprise a hydrocarbon adsorbent such as zeolite or a composite combination of a hydrocarbon adsorbent and a three-way catalyst. The hydrocarbon adsorbent has a function to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100° C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250° C., for example.
It has been desired in the art to evaluate a deteriorated state of an exhaust gas purifier in order to determine when to replace the exhaust gas purifier. It has also been desired to evaluate a deteriorated state of an exhaust gas purifier which has a hydrocarbon adsorbent. One known system for evaluating a hydrocarbon adsorbent is disclosed in Japanese laid-open patent publication No. 10-159543, for example. The disclosed system has temperature sensors disposed respectively upstream and downstream of the exhaust gas purifier which has a hydrocarbon adsorbent. The system estimates a temperature downstream of the exhaust gas purifier from the temperature detected by the upstream temperature sensor on the assumption that the hydrocarbon adsorbent is a brand-new one which has not been deteriorated. Based on the difference between the estimated temperature and the temperature detected by the downstream temperature, the system evaluates a deteriorated state of the hydrocarbon adsorbent.
However, the conventional system finds it difficult to evaluate a deteriorated state of the hydrocarbon adsorbent accurately because the temperatures detected by the temperature sensors disposed respectively upstream and downstream of the exhaust gas purifier are susceptible to various factors including the ambient temperature, etc. The system is also disadvantageous from the standpoint of cost as it needs temperature sensors both upstream and downstream of the exhaust gas purifier.
It has also been known in the art to provide a hydrocarbon sensor (HC sensor) disposed downstream of the hydrocarbon adsorbent, directly grasp an adsorbed state (adsorbed quantity or the like) of hydrocarbons adsorbed by the hydrocarbon adsorbent based on an output signal detected by the HC sensor, and evaluate a deteriorated state of the hydrocarbon adsorbent based on the adsorbed state of hydrocarbons. Generally, however, HC sensors react not to all kinds of hydrocarbons that can be adsorbed by the hydrocarbon adsorbent. The adsorbed state of hydrocarbons adsorbed by the hydrocarbon adsorbent as grasped by the HC sensor often does not sufficiently reflect the actual adsorbed state of hydrocarbons. Therefore, it is often difficult to evaluate a deteriorated state of the hydrocarbon adsorbent with accuracy. The HC sensor also makes the entire system disadvantageous as to cost because the HC sensor is relatively expensive.
The applicant of the present application has attempted to develop a technique for evaluating a deteriorated state of a hydrocarbon adsorbent with a humidity sensor disposed downstream of the hydrocarbon adsorbent or humidity sensors disposed respectively downstream and upstream of the hydrocarbon adsorbent. Specifically, according to findings of the inventors of the present application, the hydrocarbon adsorbent is capable of adsorbing not only hydrocarbons contained in exhaust gases, but also moisture contained in exhaust gases. The ability of the hydrocarbon adsorbent to adsorb moisture, i.e., the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent, is highly correlated to the ability of the hydrocarbon adsorbent to adsorb hydrocarbons, i.e., the maximum amount of hydrocarbons that can be adsorbed by the hydrocarbon adsorbent. As the hydrocarbon adsorbent progressively deteriorates, both the ability to adsorb moisture and the ability to adsorb hydrocarbons are progressively lowered in the same manner. Furthermore, as described in detail later on, it is possible to grasp the adsorbed state of moisture adsorbed by the hydrocarbon adsorbent by observing an output of the humidity sensor downstream of the hydrocarbon adsorbent or outputs of both the humidity sensors downstream and upstream of the hydrocarbon adsorbent. The humidity sensors are more inexpensive than the HC sensors or the like. Using the above humidity sensors, it is considered possible to evaluate a deteriorated state of the hydrocarbon adsorbent at a relatively low cost.
According to the finding of the inventors, if a humidity sensor is disposed downstream of the hydrocarbon adsorbent, then the humidity (detected humidity) represented by the output of the humidity sensor, i.e., the humidity downstream of the hydrocarbon adsorbent, undergoes a transition after the internal combustion engine has started to operate, as follows: Immediately after the internal combustion engine has started to operate, the moisture contained in the exhaust gases is adsorbed by the hydrocarbon adsorbent, and the humidity downstream of the hydrocarbon adsorbent is basically kept at a substantially constant low level. As the adsorption of moisture by the hydrocarbon adsorbent progresses and becomes saturated, the hydrocarbon adsorbent is no longer capable of adsorbing moisture contained in the exhaust gases, and hence the humidity downstream of the hydrocarbon adsorbent is not adsorbed by the hydrocarbon adsorbent, but increases monotonously to a higher humidity because of moisture contained in the exhaust gases that pass through the hydrocarbon adsorbent. Therefore, the timing at which the humidity downstream of the hydrocarbon adsorbent starts monotonously increasing from a low humidity level, i.e., the changing timing at which the humidity changes to a tendency to increase monotonously, is the timing at which the adsorption of moisture by the hydrocarbon adsorbent becomes saturated, i.e., the timing at which the adsorption of hydrocarbons by the hydrocarbon adsorbent becomes saturated. After the internal combustion engine has started to operate, the integrated amount of moisture supplied to the hydrocarbon adsorbent by the exhaust gases up to the above changing timing, or a value corresponding to the integrated amount of moisture corresponds to the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent at present. Therefore, after the internal combustion engine has started to operate, the integrated amount of moisture supplied to the hydrocarbon adsorbent up to the changing timing of the moisture downstream of the hydrocarbon adsorbent, or a value corresponding to the integrated amount of moisture represents a deteriorated state of the hydrocarbon adsorbent.
Therefore, in grasping a deteriorated state of the hydrocarbon adsorbent with the humidity sensor disposed downstream of the hydrocarbon adsorbent, it is important to grasp the changing timing at which the humidity detected by the humidity sensor changes from a lower humidity level to a tendency to increase monotonously to a higher humidity level.
If the hydrocarbon adsorbent is brand-new, then the humidity downstream of the hydrocarbon adsorbent after the internal combustion engine has started to operate is of a substantially constant low level immediately after the internal combustion engine has started to operate, as described above. In such a circumstance, it is relatively easy to grasp the changing timing at which the humidity downstream of the hydrocarbon adsorbent starts increasing monotonously. For example, it is possible to detect, as the changing timing, the time when the humidity detected by the humidity sensor disposed downstream of the hydrocarbon adsorbent increases a slight quantity from the start of operation of the internal combustion engine.
When the hydrocarbon adsorbent has deteriorated to a certain extent, however, the hydrocarbon adsorbent is unable to sufficiently adsorb moisture that is present therearound in the exhaust passage, and the humidity distribution in the exhaust passage tends to vary, before the internal combustion engine starts to operate. In this case, after the internal combustion engine has started to operate, the humidity detected by the humidity sensor is liable to vary. Therefore, even if the time when the humidity detected by the humidity sensor increases a slight quantity from the start of operation of the internal combustion engine is grasped as the changing timing, the grasped changing timing possibly becomes inaccurate.
As described in detail later on, in the case where the humidity sensor is disposed not only downstream of the hydrocarbon adsorbent, but also upstream of the hydrocarbon adsorbent, for grasping the amount of moisture that can be adsorbed by the hydrocarbon adsorbent and evaluating a deteriorated state of the hydrocarbon adsorbent based on the grasped amount of moisture, it is necessary to grasp the changing timing of the humidity detected by the downstream humidity sensor and also the changing timing of the humidity detected by the upstream humidity sensor. However, the above problems which occur when the changing timing of the humidity detected by the downstream humidity sensor is grasped are liable to occur when the changing timing of the humidity detected by the upstream humidity sensor is grasped. The present invention is made according to such background, and is aimed at providing a deteriorated state evaluation device of exhaust emission control equipment for accurately evaluating the deteriorated state of a hydrocarbon adsorbent with an inexpensive arrangement which employs a humidity sensor.